Not Applicable.
Not Applicable.
1. The Field of the Invention
The present invention relates generally to the replacement of the proximal tibial surface of a knee joint, and more particularly, but not entirely, to a locking system for attaching together the components of a tibial prosthesis.
2. Description of Related Art
It is known to construct a modular tibial prosthesis having a base plate and a separate articular surface component. It is advantageous to utilize a separate component for the articular surface so that it can be made from different material than the base plate. More specifically, since the base plate is implanted directly into cancellous bone in the proximal end of the tibia, it is typically made from titanium steel for reasons known to those skilled in the field, including the fact that titanium is strong, relatively lightweight, and bio-compatible. Since titanium does not operate well as a bearing surface, a separate piece, typically made from a polymer, is attached to the base plate to operate as an articulating surface in sliding, frictional engagement with the distal femur, which typically comprises a femoral component having a pair of articulating condyles.
Modular tibial prostheses utilizing a base plate and a separately attachable articular surface component require a reliable means for attaching the articular surface component to the base plate. It is important to achieve secure fixation between those components without unduly weakening one or both of them. These objectives are difficult to obtain because of the significant loads imposed on the articular surface component by the distal femur, especially in the case of a xe2x80x9cposterior stabilizedxe2x80x9d knee. The phrase xe2x80x9cposterior stabilizedxe2x80x9d knee refers to a prosthetic knee joint having a spine element extending upwardly from the tibial plate that is constrained and guided in its motion by femoral condyles and by a cam component that limits the point of maximum flexion of the knee, to compensate for removal of the posterior cruciate ligament (xe2x80x9cPCLxe2x80x9d). Conversely, the phrase xe2x80x9ccruciate retainingxe2x80x9d knee refers to a prosthetic knee joint without the cam and spine element because the knee motion is sufficiently constrained and guided by the presence of a working PCL.
Attempts have been made in the prior art to provide a modular tibial prosthesis that has an effective mechanism for attaching the articular surface component to the base plate. U.S. Pat. Nos. 5,405,396 (Heldreth et al.), U.S. Pat. No. 4,822,362 (Walker et al.); U.S. Pat. No. 5,194,066 (Van Zile) and U.S. Pat. No. 5,609,644 (Ashby et al.) each disclose attachment systems for attaching a separate articular surface component to a tibial base plate. The Heldreth et al. patent (U.S. Pat. No. 5,405,396) utilizes a dovetail structure at item 7, shown in FIG. 2, for retaining the articular surface. The Walker et al. patent (U.S. Pat. No. 4,822,362) teaches tongue-in-groove interlock ledges 28, as shown in FIGS. 1 and 5-6, to retain the articular surface component. The Van Zile patent (U.S. Pat. No. 5,194,066) uses parallel male projections shown in FIGS. 2-3 for the retaining action. The Ashby et al. patent (U.S. Pat. No. 5,609,644) uses a peg-in-slot engagement, shown in FIG. 8 (peg 7 engages within slot 17, and pin 20 engages within slot 19). None of these patents, or any of the others known to applicants, addresses the challenge of providing highly effectively resistance to movement of the articular surface component in all directions while maintaining the thickness of the articular surface component at a minimum.
The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.
It is therefore an object of the present invention to provide a tibial prosthesis locking system.
It is an additional object of the present invention to provide such a tibial prosthesis locking system that is simple in design and manufacture, and easier to use.
It is another object of the present invention, in accordance with one aspect thereof, to provide such a tibial prosthesis locking system that provides enhanced resistance to movement of an articular surface component of the prosthesis in at least four directions.
It is a further object of the present invention, in accordance with one aspect thereof, to provide such a tibial prosthesis locking system in which a thickness of an articular surface component of the prosthesis is maintained at a minimum.
The above objects and others not specifically recited are realized in a specific illustrative embodiment of a tibial prosthesis locking system. The system includes a tibial plate and an articular surface component or tibial insert. A proximal side of the tibial plate includes perimeter sidewalls with holding flanges formed thereon, and male retaining projections are formed on interior portions of the proximal tibial plate. A distal side of the articular surface component is designed to interlock with the holding flanges and retaining projections formed on the tibial plate. The male retaining projections and the holding flanges cooperatively engage with the articular surface component to provide it with resistance to movement in an anterior-posterior direction, a medial lateral direction, a rotational direction, and a vertical direction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.